Responding to eruptive transitions during the 2020–2021 eruption of La Soufrière volcano, St. Vincent

A critical challenge during volcanic emergencies is responding to rapid changes in eruptive behaviour. Actionable advice, essential in times of rising uncertainty, demands the rapid synthesis and communication of multiple datasets with prognoses. The 2020–2021 eruption of La Soufrière volcano exemplifies these challenges: a series of explosions from 9–22 April 2021 was preceded by three months of effusive activity, which commenced with a remarkably low level of detected unrest. Here we show how the development of an evolving conceptual model, and the expression of uncertainties via both elicitation and scenarios associated with this model, were key to anticipating this transition. This not only required input from multiple monitoring datasets but contextualisation via state-of-the-art hazard assessments, and evidence-based knowledge of critical decision-making timescales and community needs. In addition, we share strategies employed as a consequence of constraints on recognising and responding to eruptive transitions in a resource-constrained setting, which may guide similarly challenged volcano observatories worldwide.

-Improve the x axis labels -how can a single tick represent a whole month? Are they supposed to indicate the first day of each month? If that is the case, then that should be the label. I suggest also including some lower-level ticks, and labelling / annotating more key dates (for example start and end of explosive phase) Figure 3 -You could probably be a bit more specific in B and C and say these are epicentral plots Figure 4 -The minor ticks on the x axis are not visible -maybe make them bigger? Otherwise it is very hard to estimate what date falls where in the plots. -There is no C in this figure yet you refer to it in the text.

Reviewer #1 (Remarks to the Author):
Overall This was an interesting paper on how an observatory translates science into actionable forecasts and hazard assessments, and how it communicates these with the public. This is a topic that deserves more attention, as it is a critical (perhaps the most critical) part of an eruption response. As the majority of attention in the literature is usually devoted to fundamental science, this paper helps fill in an important gap in the field. My main comment is that the article should spend more time on the broader implications of this eruption response, and how it might inform crisis response elsewhere -right now the scope of the paper is too limited. My other comments are all minor, and include reading through the manuscript to fix an assortment of awkward sentences.

Main comment
In my reading, the paper details the 2020-2021 La Soufriere response and the successful aspects of it, but stops short of explicitly presenting broader implications from the experience and how these insights can be applied elsewhere. I'm assuming for this journal, in particular, there should be a significant portion of the paper devoted to what this response teaches us in a broader sense. The introduction does a good job of setting up the motivation, but the discussion and conclusion seem to fall short on relating this study to the outside world. These sections invoke some general statements about volcano monitoring, but I think the reader will want more explicit or detailed links between this study and other volcanoes. I was expecting some discussion of how this response might relate to eruption crises at other volcanoes (right now there's very little mention of other volcanoes or eruptive crises), and how the lessons learned here might be applied in other scenarios.
Specific comments • Line 234: Here it is stated that the elicited probabilities rose dramatically, and it would be helpful to note briefly the reason -the appearance of banded tremor, correct? • Line 237: It would be helpful here to note again the dates when the explosions occurred.
• Line 262: The comment about harmonizing the messaging between the observatory and emergency responders is an important one. It brings to mind recent activity we were involved in here in Hawai`i, and this paper might be relevant in how communication can be done in the midst of an eruption crisis: https://pubs.geoscienceworld.org/books/book/2116/chapter/115281994/Communication-strategyof-the-U-S-Geological • Line 335: It's interesting that the flushing of early magma followed by fresher magma, with the associated increase in eruption vigor, is observed in this eruption and also in other environments like Kilauea. The Gansecki et al. paper below also describes how this shift in composition implied a change in eruptive vigor, and how this concept was communicated with emergency responders as the events unfolded: https://www.science.org/lookup/doi/10.1126/science.aaz0147 • Line 345: This is an example of an awkward sentence, and there are multiple examples of this throughout the paper. One way to avoid sentences like this is to read through the paper out loud and find where the reading gets tripped up. I know I was getting tripped up a few places. • Line 360: The wording here is unclear, especially for item (i) • Line 580: This is vague, and I'm not sure exactly what is meant here.
• Figure 10 is an excellent communication summary. It's a busy graphic, but it contains the needed information. We wish to thank the reviewers and the editor for their time and consideration of our manuscript. We have addressed the referees" comments as detailed below (referee comments shown in italics). All minor corrections have been accepted, text (see "track changes" version of the resubmitted manuscript) and figures and tables have been modified to accommodate the suggestions of the reviewers. The line numbers mentioned in the responses below refer to the changes marked version of manuscript.
We are particularly grateful for the editorial steer around the contrasting reviews. The key focus here is on the "crisis science" -which is what reviewers 1 and 3 have pointed out. This focus perhaps speaks more clearly to the ambition of Nature Communications than the tradition of scientific focus some readers might have for "Nature". We are grateful for the opportunity to help achieve that ambition. However, from reviewer comments, this needed to be clarified and developed in the responses. We have focused on strengthening relevance to other situations worldwide, particularly for volcanoes that experience transitions from effusive to explosive eruptions and the key findings of relevance to "science into action". The detailed comments on their reviews (and those of Reviewer 2) have helped with this. An important dimension here, which we now develop more fully in the discussion is pushing the frontier of this work in a resource-constrained setting.
We summarise here the contributions to this: (a) in the introduction we have more clearly introduced the concept of crisis science, its relationship with operational volcanology and the value of the La Soufrière experience to the wider volcanic community.

Reviewer #1:
Review of "The 2020-2021 eruption of La Soufriere volcano, St. Vincent: Monitoring and scientific response" by Joseph et al. Review by Matt Patrick, USGS-HVO, mpatrick@usgs.gov Overall This was an interesting paper on how an observatory translates science into actionable forecasts and hazard assessments, and how it communicates these with the public. This is a topic that deserves more attention, as it is a critical (perhaps the most critical) part of an eruption response.
As the majority of attention in the literature is usually devoted to fundamental science, this paper helps fill in an important gap in the field. My main comment is that the article should spend more time on the broader implications of this eruption response, and how it might inform crisis response elsewhereright now the scope of the paper is too limited. My other comments are all Web: www.uwiseismic.com | Tele: +1-868-662-4659 | E-mail: src@sta.uwi.edu minor, and include reading through the manuscript to fix an assortment of awkward sentences.
Main comment  In my reading, the paper details the 2020-2021 La Soufriere response and the successful aspects of it, but stops short of explicitly presenting broader implications from the experience and how these insights can be applied elsewhere. I"m assuming for this journal, in particular, there should be a significant portion of the paper devoted to what this response teaches us in a broader sense. The introduction does a good job of setting up the motivation, but the discussion and conclusion seem to fall short on relating this study to the outside world. These sections invoke some general statements about volcano monitoring, but I think the reader will want more explicit or detailed links between this study and other volcanoes. I was expecting some discussion of how this response might relate to eruption crises at other volcanoes (right now there"s very little mention of other volcanoes or eruptive crises), and how the lessons learned here might be applied in other scenarios.
We have attempted to address this in the revised manuscript by refocusing the paper, particularly in the • Line 335: It"s interesting that the flushing of early magma followed by fresher magma, with the associated increase in eruption vigor, is observed in this eruption and also in other environments like Kilauea The Gansecki et al. paper below also describes how this shift in composition implied a change in eruptive vigor, and how this concept was communicated with emergency responders as the events unfolded: https://www.science.org/lookup/doi/10.1126/science.aaz0147 (see lines 381 -383 and following discussion lines 391 -404 on changes in eruptive activity, as well as section 2.3 on communication strategies throughout the eruption).
• Line 345: This is an example of an awkward sentence, and there are multiple examples of this throughout the paper. One way to avoid sentences like this is to read through the paper out loud and find where the reading gets tripped up. I know I was getting tripped up a few places. Paragraph deleted; idea now included in lines 402 -404.
• Line 360: The wording here is unclear, especially for item (i) See revised sentences (lines 424 -431).
• Line 580: This is vague, and I"m not sure exactly what is meant here. See revised sentence (lines 707 -708).
• Figure 10 is an excellent communication summary. It"s a busy graphic, but it contains the needed information. Figure 10 has been revised.

Reviewer #2:
Reviewer #2 (Remarks to the Author): According to its title, this manuscript is an account of the scientific response during the 2020-21 eruption at St. Vincent island. The account describes the implemented monitoring system, the observations recorded, and the communication with stakeholders (public officials and decision makers, the media and the public). The volcano was poorly monitored, but when the unrest started, the monitoring system was improved. The scientific response was such as to lead to quick evacuation of the areas subject to risk, and no losses were reported.
The account tells a story which is not dissimilar from others characterizing active volcanoes in settings with limited resources. The scientific content is rather low and far from the standards characterizing papers published in journals with high impact. The reflection on managing volcanic risk in a not highly developed setup does not bring much, as similar contents are found in other such accounts (e.g., those relating to a number of recent eruptions from Indonesia published in specialized journals). Sincerely, I do not think this account is of any interest sufficient to justify a publication in Nature Communications. Below I report more specific comments.
In general, the risk response organization does not emerge clearly, as it is not fully clear who is responsible for what. For example, at line 536 it is said that "Protocols to alert the whole population […] were developed with the target communities etc.". That does not clarify who developed the protocols for alerting the population, what was the related role of scientists, and finally if scientists are expected to alert themselves or have a role in alerting the population.
The adopted VALS (reported in the Supplementary Information) does not help, as it presents direct relationships between scientific evaluations and actions on the population (once you have the former, the latter is also determined); contributing to depict a system where scientists may effectively contribute to make decisions for the population. That may be the case, but it would be worth saying it clearlyor if it is not, then avoid any source of possible misunderstanding.

Sentences revised to avoid possible misunderstanding (lines 633 -635). The word protocol was changed to procedures (line 641) to better reflect the meaning of the sentence, as it was not meant to refer to the actual alerting protocols, but the actions taken by citizens in the Red Zone when an evacuated order/alert is given by the local authorities (NEMO) -lines 471 -475.
Line 90: UWI is introduced without definition (I guess it is University of West Indies).

Correct. Change made in line 102.
Line 121: Fig. 1 is quoted in relation to an "increase in seismicity in November 2020"; however, the figure does not include that period. Sentence revised (see lines 128 -130) and reference to Figure 1 removed. .
Line 128: what is the meaning of "similar" in this context? The frequency of the second group of events is up to one order of magnitude smaller, which does not seem to justify the conclusion that they are "similar". On the other hand, there are no other characteristics reported, so the statement remains unclear.

Corrected: our analysis shows that the events share the same properties of low frequency events reported in literature (see lines 133-135).
Line 130. Fig. 3B is quoted in relation to "Hypocenters" which "delineated a NW-SE structure". However, the figure does not show hypocenters, only epicenters.

Paragraph revised (see lines 131 -138).
Fig. 4A. The claimed "banded tremor" cannot be clearly appreciated under the thick lines drawn to evidence it. That is reported as the suggestion for "an imminent explosive phase onset"; this seems to deserve a clearer picture and more thorough discussion.

More information and references were included in the text where banded tremor is mentioned. We highlighted the fact that this tremor is related to shallow fluid excitations (gas and hydrothermal), which is a sign of a transition to explosive phase (due to the lack of gases involved during the effusive phase). Lines 139 -142.
Web: www.uwiseismic.com | Tele: +1-868-662-4659 | E-mail: src@sta.uwi.edu Line 143. The claimed "same stable frequency content" from the spectrograms is unclear from the figure, and not supported by any quantitative analysis.

A spectrogram is a recognized form of quantitative analysis and Figure 4 was revised to depict the characteristics referred to more clearly. Sentence revised (lines 151 -153).
Line 145. There is a claimed "exponential decay" in Fig. 4, but no explanation on how such a conclusion is reached. Figure 4 zoomed in and RSAM included, each explosion has an exponential decay, it is a direct observation.
Line 154. The note on the shallow (about 500 m) character of the magma intrusion relates to the recorded amplitude, which looks weird without analysis of the geometry of the deformation. Please explain. Related to this point: at line 161 another phase of deformation is said to be associated to a 6-7 km deep source. Is the geometry of the deformation well constrained, and if so, why not showing it?
The near-field deformation at the start of the effusive phase, which was only observed with InSAR data (there were no GPS station near the crater at the time), was best modelled using an Okada dislocation source, which is detailed in Figure 5"s caption. Conversely, modeling of the explosive phase using cGPS data, was best characterised using a Mogi point source model. We modified the text to ensure the two time periods and their corresponding deformation source modelled are now clearer to the reader (lines 176 -178). More detailed modeling to constrain the source geometry will be addressed in another scientific paper and so we opted not to show this here due to the broader scope of this paper.
The following line 162 reports an estimate of the "overall erupted volume" (over about 10 days) which appears to compare with the deformation "in the first 24 hours…"; what is the relationship between the two estimates, which refer to time spans differing by one order of magnitude?

Sentence comparing volume of erupted material removed over different time spans was removed (lines 178 -180).
Lines 167 and followings (beginning of section 1.4), and Fig. 2. Geochemical ratios are reported without reference units. Are these mass, molar, volume or other ratios? These are ratios of concentrations in parts per million (ppm) (lines 184 -189). See methodology section, lines 558 -568 for more information on the geochemical measurements using MultiGAS.
Line 179. The inferred viscosity is surprisingly high, comparable with that of dry rhyolite and definitely not typical for basaltic andesites. The reported reference for typical viscosities at La Soufriere is that of Cole et al. 2019, but those authors do not even mention viscosity in their paper. Please explain.
The correct inferred viscosity of 10 11 Pa.s, the 11 was not in superscript in the original submission hence the misinterpretation by the reviewer, however, the sentence was removed (line 204 -206).
Section 2 on Crisis Response. I have outlined above my major concern relating to unclear definition of roles and responsibilities. I add that this section includes a higher proportion of sentences whose English is poorly effective contributing to generate confusion. In responding to your broader comments, we have substantially improved the awkward phrasing and considerable revisions have been made to the entire manuscript.
Line 223. The reported source of uncertainty ("imperfect understanding") is only the epistemic component of uncertainty. There is another component which is not mentioned and can be equally or even more relevant. This is the aleatoric component of uncertainty, generated by the complex behavior of highly non-linear volcanic systems. Overlooking the aleatoric component of uncertainty may be dangerous when interpreting complex sequences of data, as it may lead to over-interpretations and poorly justified conclusions.
Web: www.uwiseismic.com | Tele: +1-868-662-4659 | E-mail: src@sta.uwi.edu We have now significantly improved this and referred to a relevant paper that deals with uncertainty but also more clearly stated the particular relevance to how uncertainty in eruption forecasting was treated during the eruption (lines 260 -266).
Line 232. Reference to "central value" is ambiguous, as there are many measures of the central tendency of a distribution. The median is cited two lines above, so I guess that"s the referred quantity? Please clarify.
Central value changed to median value (see line 271 and 273) and is the quantity referenced.
Line 234. Here it is said that the elicitation process led to a strong increase (from 20 to 60%) in the (median? Please clarify) probability of explosive activity in the morning of 8 April. What were the observations that led to such a dramatic change in the evaluations by the elicited experts? Sentence revised to state median value. The observations that led to the change was the appearance of banded tremor (see lines [272][273], which was interpreted as indicating an imminent explosive phase, with a source attributed to excitation of shallow gas and fluid pockets (lines 139 -142). Fig. 2 seems to show that no major changes occurred on April 8 (VT seismicity had increased in the previous days and does not seem to show anything major on 8 April). Low amplitude tremor is reported, is it the major cause of the jump in probability? Correct, banded tremor reported and depicted in Figure 4, was the major cause of the jump in probability (see previous comment above).
Line 234: SVG (Saint Vincent and the Grenadine, I guess) is not previously introduced.

Revised and introduced in line 84.
Line 243 (and others similar): put a space between numbers and their dimensions.

Paragraph revised and sentence removed (lines 278 -284), however, the information on visual observations in the early days of the explosive transition is now reported in lines 236 -240.
Discussion section. This is all highly speculative, the kind of conceptual modeling any volcanologist extracts from sequences of data and observations, in this case with relatively limited data. This main objective of the manuscript is to show how an observatory translates science into actionable forecasts and hazard assessments and how it communicates these with the public as part of an ongoing eruption response. This is something that is currently a gap in the literature. The Discussion section has been revised to reflect the broader implications of this eruption response, and how it might inform crisis response elsewhere, particularly for responding to eruptive transitions thereby widening the scope of the manuscript (see detailed response to main comment by Reviewer #1 above).

Reviewer #3:
Lindsay review of "The 2020 -2021 eruption of La Soufrière volcano, St. Vincent: Monitoring and Scientific Response" by Joseph et al.

Overview
This paper presents an overview of the monitoring data during the 2020 -2021 eruption of La Soufrière volcano in St. Vincent, and how the data, combined with prior knowledge and experience of the volcano, were used to inform decision-making, crisis communication and assessment of risk. This is a very interesting manuscript that provides new insights into a very recent eruption, and that highlights some of the challenges of responding to volcanic crises, especially in resource-constrained contexts. I think it will be of interest to a wide audience as it provides a comprehensive summary of the eruption, the monitoring data that were obtained, and the actions that were takenincluding the development of an evolving Web: www.uwiseismic.com | Tele: +1-868-662-4659 | E-mail: src@sta.uwi.edu conceptual model. Overall, it is a well-written paper and my comments are generally quite minor. I understand that papers published by Nature Communications "represent important advances of significance to specialists within each field". Although this manuscript doesn"t present advances as such, I am of the opinion it is extremely important to document such eruptions, eruption responses, and the link between data streams and evolving conceptual models. If required for publication in Nature Communications I am sure the manuscript could be tweaked so that its focus is more on the novel aspects. See comments above in response to Reviewer #2 about the importance of thinking about science in context in the case of a volcanic eruption and we present a new and exciting dataset in this regard.
General comments and suggestions: I found it unusual that the hazard zones in the hazard map were labelled in the key as red, orange, yellow and green, instead of an explanation of what the zones mean. That"s the equivalent of a key to a dotted area on a map just saying "dots" in the legend. Is this really the map that was used? It may be that the population on St Vincent are so familiar with the hazard zones that it is sufficient to just show the colours with no explanationbut I think for an international audience the original descriptions of the zones are required somewhere.
Agreed. Changes made to the label and caption of Figure 1 to provide a description of the zones.
I wonder if it would be worthwhile providing the actual results of your life-safety risk assessment and how these evolved during the crisis-and an explanation for how this affected decisions made to undertake field work? This would be of interest to other observatories and agencies I am sure.

Revised to include an example of the results of the life-safety risk assessment in the context of the initial period of vital network-strengthening in high-risk areas and the operational impact has been added (lines 457 -459), additional supplementary figure now included to depict the results of the life-safety risk assessment.
Alsoin the risk assessment section on page 23 it seems you really only describe in detail the expert elicitation related to the evolving eruptive processes (ie the second approach)which is not a risk assessment in itself. Maybe make this section clearer by providing a bit more information about how the results of the expert elicitation fed into any actual risk assessments, including for example how (if at all) the expert elicitation that you describe here feeds into the life-safety risk assessment.  -Improve the x axis labelshow can a single tick represent a whole month? Are they supposed to indicate the first day of each month? If that is the case, then that should be the label. I suggest also including some lower-level ticks, and labelling / annotating more key dates (for example start and end of explosive phase) X axis revised to include suggestions.    Supplementary material -It would be good to have more detail in the captions about what we are actually looking at in these two figures. What was the first flyer used for? Who was the audience? In the second figurewho is in all the figures? The journal will likely require written permission from everyone in the photographs in order to publish them. What are we looking at exactly? Are we looking at screen shots of radio interviews? You tube videos? Please explain in the caption. This addition to the Supplementary material was removed from the revised submission.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): I have read the revised version of the manuscript, and I think it is much improved over the original submission. Specifically it addressed my main concern on the lack of broader impact, but this has been fixed with a new section in the discussion about anticipating effusive/explosive transitions. Overall I don't see any need for further revisions.
Reviewer #2 (Remarks to the Author): I have read the responses by the authors to my comments, and the revised manuscript. Although I appreciate the authors' efforts, my conclusion remains that this manuscript is 1) out of scope for this journal as it does not address any "important advances of significance to specialists", nor it aims at doing so; 2) not original or novel, as it tells a story which is not dissimilar from that of other cases of claimed success in managing volcanic risk at poorly (or relatively poorly) monitored volcanoes. As from my previous conclusion, as a reader I would ask myself why this particular account is given the status of a highly impacting journal publication, instead of being more conveniently hosted in one specialized journal as for other similar accounts.
The arguments above do not refer to scientific or technical flaws but to convenience and appropriateness for the manuscript to become a Nature Communications paper. As such, any decision thereby is more appropriately made by the journal Editors.
Reviewer #3 (Remarks to the Author): I have had the chance to review the revised manuscript and I think you have done a good job addressing all the reviewers' concerns. Although it is a sometimes a challenge to justify publications that report on an eruption crisis rather than a new an exciting scientific discovery, such documentation of eruptions is extremely valuable. In your revisions and response to reviewers' comments you have made efforts to more clearly point out the scientific contribution your work is making. Well done!
The only think I couldn't find in the revised manuscript were the Figure captions. It may be that there are there and I just couldn't see them -but maybe double check.